Demands for higher integration, higher operation speed, higher performance, and lower power consumption have been more stringent, and in response to the demands, a transistor using an SOI substrate has attracted an attention as effective means that can take the place of a bulk transistor. Higher operation speed and lower power consumption can be expected more in the transistor using the SOI substrate than in the bulk transistor because an insulating film is formed over a semiconductor film in the transistor using the SOI substrate, and accordingly parasitic capacitance can be reduced and generation of leakage current flowing through the substrate can be suppressed. In addition, since the semiconductor film used as an active layer can be formed thin, a short channel effect can be suppressed; thus, an element can be miniaturized, and higher integration of a semiconductor integrated circuit can be realized accordingly. Moreover, since the transistor using the SOI substrate is completely latch-up free, there is no possibility that the element is broken due to heat caused by latch-up. Furthermore, the transistor using the SOI substrate does not need element isolation unlike the bulk transistor; thus, the transistor using the SOI substrate has advantages in that the distance between the elements can be shortened and higher integration can be realized.
As one of manufacturing methods of an SOI substrate, there is a method in which a semiconductor film is bonded to a substrate with an insulating film interposed therebetween, such as UNIBOND (registered trademark) typified by Smart Cut (registered trademark), ELTRAN (epitaxial layer transfer), a dielectric separation method, or a PACE (plasma assisted chemical etching) method. The above-described bonding methods make it possible to form a high-performance integrated circuit, in which a single crystal semiconductor film is used, over an inexpensive substrate.
As one example of the semiconductor device using the SOI substrate, one disclosed by the present applicants is known (Reference 1: Japanese Published Patent Application No. 2000-012864).